The present invention relates generally to polymeric articles containing fluorescent dyes. More particularly, the invention concerns dye-stabilized polyvinyl chloride compositions, and articles therefrom, useful for making flexible retroreflective sheeting that has superior fluorescence durability and improved retention of retroreflectivity.
In the highway construction industry, fluorescent orange warning signs are used to alert drivers of on-going construction activity. The bright color and/or the fluorescent material attracts the driver""s eye to the sign. Typically, these signs are fabricated by adhesive lamination of retroreflective sheeting to a rigid aluminum or wood sign substrate. However, such a rigid construction has two primary problems. First, the wood or aluminum substrate makes these signs bulky and awkward to move, especially for temporary highway construction activities. Second, such signs can present a potentially deadly hazard to construction zone workers. When hit by a vehicle moving at high speeds, these rigid signs become projectiles capable of severely injuring a worker. Hence the need for soft, flexible signs which can easily be rolled-up and moved, and which do not present a projectile hazard.
Fluorescent roll-up signs fabricated from flexible polyvinyl chloride (xe2x80x9cPVCxe2x80x9d) are known in the art. Unfortunately, when used to manufacture microprismatic retroreflective films, flexible PVC suffers from two serious drawbacks: loss of retroreflectivity, and loss of fluorescence. Loss of retroreflectivity is due in part to the fact that PVC is a soft, flexible material. Unlike polymethyl methacrylate or polycarbonate microprisms (used in traditional sheeting materials) which are rigid and inflexible, microprisms formed in plasticized PVC are subject to deformation when the sheeting material is exposed to high tension or pressure as is typically the case under the stress conditions encountered in the production, handling and ultimate roadway use of such sheetings. Typically, if the microprism cube corner angles change by more than about 0.01xc2x0, the retroreflective levels will be significantly reduced.
Losses in retroreflectivity can further be attributed to the use of plasticizer compounds in the manufacture of PVC film. Plasticizers are typically added to PVC resin to make the resin more flexible. However, most plasticizers will eventually migrate from the PVC film. When the flexible PVC is used for retroreflective applications, such migration can result in retroreflectivity losses due to slight disfiguration of the microprisms as the plasticizer exudes from the film and changes the film""s overall material composition. This phenomenon is particularly noticeable in outdoor applications where the material may be exposed to elevated temperatures during the summer months. The plasticizers commonly used in PVC retroreflective film are low molecular weight monomeric plasticizers (typically  less than 700 grams/mole) and often from the phthalate family, such as dioctyl phthalate or diisononyl phthalate. Given the problems noted above, there is presently a need to provide PVC reflective sheeting capable of retaining more of its retroreflectivity over time.
A further problem in fluorescent PVC roll up signs is that ultraviolet exposure causes the fluorescence to fade very quickly, typically in two to three months. To circumvent this problem, many manufacturers create fluorescent orange PVC roll-up sheets by combining two different layers of PVC. A top (outer) layer of the flexible PVC is colored with a fluorescent pink colorant. A lower layer having reflective elements is colored with a non-fluorescent orange colorant. The combination of these two layers produces a brilliant fluorescent orange color. After the pink flourescent layer fades (which typically occurs in 2-3 months of outdoor exposure), a non-fluorescent orange sign will remain. Although this approach for extending the useful life of flexible highway signs has met with some acceptance, it still does not address the fundamental problem of how to achieve a more stable (and therefore longer-lived) fluorescence in retroreflective PVC roll-up films when fluorescent dyes are incorporated directly into the retroreflective PVC layer.
The present invention provides a fluorescence-stabilized polymeric composition comprising: polyvinyl chloride resin; a fluorescent dye selected from the group consisting of perylene imide fluorescent dyes; an ultraviolet absorbing compound selected from the group consisting of benzophenone ultraviolet absorbers; and a light stabilizing compound selected from the group consisting of polymeric hindered amine light stabilizer compounds having molecular weight greater than about 1500. The combination of benzophenone UV absorbers and polymeric HALS lends surprising stability to the perylene imide dyes.
The invention is further directed to a polymeric film suitable for fabricating retroreflective sheeting, comprising polyvinyl chloride resin and a polymeric plasticizer having molecular weight greater than about 1000, and preferably in the range of about 2000 to about 8000. Particularly preferred are polymeric plasticizers having molecular weight in the range of about 2500 to about 5000. It has been discovered that the use of a polymeric plasticizer can substantially minimize the retroreflectivity losses which flexible PVC signs typically exhibit over time.
In still a further aspect the invention is directed to a flexible road sign capable of being manually rolled and unrolled by a user, wherein the sign comprises a fluorescent, retroreflective polymeric sheet. The retroreflective sheet comprises a polyvinyl chloride resin having an intrinsic viscosity in the range of about 1.02 to about 1.14; a polymeric plasticizer having molecular weight in the range of about 2000 to about 8000; a fluorescent perylene imide dye; an ultraviolet absorbing benzophenone compound; a polymeric hindered amine light stabilizing compound having molecular weight greater than about 2000; and further comprising a plurality of retroreflective elements.
In a particularly preferred embodiment the invention is a flexible, rollable fluorescence-stabilized retroreflective road sign comprising a retroreflective, flourescent sheet, where the retroreflective sheet comprises; (i) polyvinyl chloride resin having an intrinsic viscosity in the range of about 1.08 to about 1.10; (a) a plasticizer mixture comprising (1) a polymeric plasticizer selected from polyester glutarate, adipate or sebacate plasticizers having molecular weight in the range of about 2500 to about 4200, and (b) a monomeric plasticizer selected from phthalate plasticizers having molecular weight less than about 1000; wherein the polyester plasticizer constitutes about 25 to about 45 percent by weight of the plasticizer mixture; and the plasticizer mixture is present in the polymeric sheet in an amount ranging from about 30 to about 35 parts per hundred by weight of said polyvinyl resin in the sheet; (iii) a mixture of perylene imide fluorescent dyes comprising BASF Lumogen F Red 300 and BASF Lumogen F Orange 240, wherein the dye mixture constitutes about 0.05 to about 0.5 percent by weight of the sheet; (iv) an ultraviolet absorbing compound comprising 2-hydroxy-4-n-octoxybenzophenone present in an amount constituting about 0.2 to about 4 percent by weight of the sheet; (v) a polymeric hindered amine light stabilizing compound comprising the polymerization product of dimethyl succinate and 4-hydroxy-2,2,6,6-tetramethyl-1-piperidine and having molecular weight greater than about 2000, wherein the light stabilizing compound constitutes about 0.1 to about 3 percent by weight of the sheet; and (vi) a plurality of cube-corner retroreflective elements.
In still a further embodiment, the invention provides a polymeric retroreflective film comprising polyvinyl chloride resin and a plurality of retroreflective elements in the form of corner-cube microprisms, wherein the film exhibits an xe2x80x9cAverage Percent Retention of Retroreflectivityxe2x80x9d (as determined herein) of about 60% or greater.
The invention provides a number of significant advantages. For the first time, fluorescent PVC films suitable for roll-up signs are now able to exhibit greatly enhanced fluorescence durability. In addition, PVC films in accordance with the invention show xe2x80x9cAverage Percent Retention of Retroreflectivityxe2x80x9d (as described in the Examples) of about 60% or greater. The enhancements in fluorescence and retention of retroreflectivity made possible by the invention will substantially extend the useful life of flexible PVC roll-up signs beyond that currently attainable. Moreover, it will no longer be necessary to laminate additional fluorescence-protective polymeric layers onto PVC reflective sheeting to compensate for the poor fluorescence stability such products have exhibited to date. Further advantages will become apparent hereinafter to those skilled in the art.
Although light absorbers and stabilizers have commonly been employed in the past to improve the light stability of flexible non-dyed PVC films, until the present invention the art has not found a way to prevent fluorescent dyes from rapidly losing their fluorescent properties when such dyes are incorporated directly into PVC films. Accordingly, the present invention is based in part on our discovery that the fluorescence stability of perylene imide dyes in PVC articles can be markedly enhanced by incorporating into a PVC formulation used to make the article the combination of a benzophenone UV absorber and a polymeric hindered amine light stabilizer (xe2x80x9cHALSxe2x80x9d) compound. Our discovery is rather surprising and unexpected for a number of reasons. First, other workers in this art have taught that HALS compounds having molecular weights exceeding 1000 grams/mole performed poorly in the stabilization of fluorescent thioxanthene dies in PVC film (see PCT WO99/20688, published Apr. 29, 1999). Secondly, we have found that the HALS/benzophenone combination works substantially better than the combination of HALS and a benzotriazole UV absorber. This is quite unexpected considering that the benzotriazoles, per se, are recognized in the art as better UV absorbing compounds than the benzophenones. Finally, there is a significant body of scientific literature suggesting that HALS compounds are incompatible with PVC. In particular, the art has recognized that amine groups tend to accelerate the degradation of PVC through dehydrochlorination of the vinyl polymeric backbone during high temperature processing.
The flourescent dyes used in the present invention are dyes selected from the well-known general class of dyes known as perylene imides. Examples thereof are the perylene imide dyes having the general formula: 
where R1 is C5-C20-alkyl which may be interrupted by an oxygen atom, or is phenyl which is monosubstituted or poly substituted by C1-C13 alkyl or C1-C13-alkoxy; and
R2 is hydrogen, chlorine, phenoxy, or halogen-, C1-C4 alkyl, or C1 to C4 alkoxy-substituted phenoxy.
Preference is given to dyes having the above structure, and blends thereof, in which R1 is selected from xe2x80x94C3H6xe2x80x94OC2H5 and 2,6-diisopropylphenyl; and R2 is selected from H, Cl and phenoxy.
Examples of such dyes are found in U.S. Pat. Nos. 5,736,596 and 5,470,502, which are incorporated herein by reference.
Further perylene imide fluorescent dyes contemplated for use in the invention are the dyes and dye blends disclosed in U.S. Pat. Nos. 5,736,596; 5,470,502; 5,808,073; 4,286,094; 4,115,386; 4,230,858; 5,920,429; 5,710,197; 5,674,622; 5,672,643; 5,605,761; 5,754,337; 5,387,458; and WO9921937 all of which are incorporated herein by reference in their entirety.
While any suitable color of perylene imide fluorescent dye may be used in the invention, the orange, red and yellow perylene imide fluorescent dyes are particularly well-suited to reflective PVC road sign applications because of their enhanced visibility under daytime as well as nightime driving conditions. Particularly preferred are the fluorescent orange and red dyes.
Specific examples of a perylene imide fluorescent dyes particularly preferred for use in the invention are xe2x80x9cLumogen F Orange 240xe2x80x9d and xe2x80x9cLumogen F Red 300.xe2x80x9d which are commercially available from BASF.
The amount of dye to be used in a fluorescent PVC composition according to the invention is in the range of about 0.05 to about 0.5 percent, and preferably in the range of to 0.20 to about 0. 25 percent based on the total weight of the PVC formulation used to fabricate a flexible PVC sheet.
Benzophenone UV absorbing compounds are well known in the art. See e.g., U.S. Pat. No. 4,980,512. Although any benzophenone UV absorber may be used in the invention, particularly suitable are those having the general formula: 
where R1 is C1 to C8 alkyl; and R2 and R3 are independently H, OH, C1-C8 alkoxy, C1 to C4 alkyl or halogen. Preferably R2 and R3 are H, and R1 is selected from -n-C8H17 or xe2x80x94OCH3. Examples of suitable benzophenone compounds are 2-hydroxy-4-n-octoxybenzophenone commercially available from Great Lakes Chemical Corporation under the trade name xe2x80x9cLowilite 22,xe2x80x9d and also from Ferro Corporation under the trade name xe2x80x9cUV-Chek AM-300xe2x80x9d. Another suitable benzophenone is 2-hydroxy-4-methoxybenzophenone commercially available from Cytec, Inc. under the trade name xe2x80x9cCyasorb 351.xe2x80x9d
The amount of benzophenone UV absorber suitable for use in a fluorescent PVC composition according to the invention is in the range of about 0.2 to about 4 percent, and preferably in the range of about 1.25 to about 2.5 percent based on the total weight of the PVC formulation used to fabricate a fluorescent PVC article.
Hindered amine light stabilizer (xe2x80x9cHALSxe2x80x9d) compounds suitable in the present invention are polymeric having molecular weights of about 1500 and greater. The term xe2x80x9chindered amine light stabilizerxe2x80x9d is intended to mean any additive used to stabilize fluorescent dyes where the compound has at least one secondary or tertiary amine. Examples of HALS compounds which are particularly suitable for use in the invention are the polymeric hindered amine compounds obtained by polymerizing in a known manner dimethyl succinate with 4-hydroxy-2,2,6,6-tetramethyl-1-piperidine ethanol. Such HALS compounds have the general formula: 
In the above formula, xe2x80x9cnxe2x80x9d is preferably selected such that the polymeric HALS has a molecular weight of at least about 2000, and most preferably greater than about 2500. Specific examples of such HALS compounds are xe2x80x9cLowilite 62xe2x80x9d sold by Great Lakes Chemical Corporation and xe2x80x9cTinuvin 622xe2x80x9d sold by Ciba-Geigy Corporation both having molecular weight above 2500. Although other workers in this art have found these polymeric HALS compounds do not work as fluorescent stabilizers for thioxanthene dyes in PVC film (see PCT W099/20688) we have found the opposite is true for perylene imide dyes, provided, however, that the HALS compound is combined with a benzophenone UV absorber as described above.
The amount of HALS compound used in a fluorescent PVC composition of the present invention is in the range of about 0.1 to about 3 percent, and preferably in the range of about 0.4 to about 1 percent based on the total weight of the PVC formulation used to fabricate a fluorescent PVC article.
The present invention is further based in part on the discovery that a flexible PVC sheet having reflective cube-corner microprisms can retain a much greater degree of retroreflectivity after accelerated aging if the formulation used to fabricate the sheet incorporates a polymeric plasticizer instead of, or in combination with, the conventionally used monomeric plasticizers. Prior to the present invention, monomeric plasticizers have been used in the fabrication of flexible PVC sheeting for reflective road sign applications. The monomeric plasticizers typically used in such sheeting (e.g., dioctyl phthalates and/or diisononyl phthalates) have had molecular weights less than about 700 grams per mole.
While any polymeric plasticizer having molecular weight greater than about 1000 may be used in the invention, those having a molecular weight in the range of about 2000 to about 8000 grams per mole are preferred. Polymeric plasticizers having molecular weight in the range of about 2000 to about 5000, and most preferably in the range of about 2500 to about 4200 are particularly well-suited for fabrication of reflective PVC sheeting having enhanced retention of retroreflectivity. A preferred class of polymeric plasticizers is the aliphatic polyesters, e.g., the polyester glutarates, the polyester adipates, and polyester sebacates, and the like. Compounds of this type are readily available commercially from, for example, C.P. Hall Company. Preferred polymeric plasticizers used in the present invention are selected from the polyester glutarates and polyester adipates sold under the trade names xe2x80x9cPlasthall P-550xe2x80x9d, xe2x80x9cPlasthall P-645xe2x80x9d, and xe2x80x9cPlasthall P-7046xe2x80x9d by C.P. Hall Company of Chicago.
While the present invention contemplates PVC formulations for reflective sheeting in which the plasticizer may consist essentially entirely of polymeric plasticizers, it is preferred for optimum cost, processing and properties of PVC flexible sheets according to the invention to use the combination of a monomeric plasticizer, for example the alkylphthalate plasticizers having molecular weight under about 1000 (e.g. Exxon""s xe2x80x9cJayflex L11Pxe2x80x9d monomeric plasticizer), with a polymeric plasticizer (e.g., C.P. Hall and Company""s xe2x80x9cPlasthall P-550xe2x80x9d, xe2x80x9cPlasthall P-645xe2x80x9d and/or xe2x80x9cPlasthall P-7046xe2x80x9d) having molecular weight in the range of 2000 to about 5000. In this preferred combination of monomeric and polymeric plasticizers, the polymeric plasticizer should constitute about 25 to about 45 percent by weight of the monomeric/polymeric plasticizer mixture.
The total amount of plasticizer used in the invention (either entirely polymeric, or as a mixture of monomeric and polymeric plasticizers) is preferably in the range of about 18 to about 25 percent, and preferably in the range of about 20 to about 23 percent, based on the total weight of the PVC composition used for fabricating a PVC reflective sheet. Relative to the amount of PVC resin in a reflective sheet formulation, the total amount of the plasticizer in the PVC composition is preferably in the range of about 30 to about 35 parts per hundred parts of the PVC resin.
The improvement in retroreflectivity made possible by the present invention""s use of polymeric plasticizer may be expressed in terms of xe2x80x9cPercent Retention of Retroreflectionxe2x80x9d. Briefly, Percent Retention of Retroreflection of a retroreflective sheet is determined by taking an initial measurement of reflectivity made at a -4 degree entrance angle and at a 0.2 degree observation angle. Following this measurement, the reflective PVC sheet is subjected to an elevated temperature for a predetermined amount of time. Then the retroreflectivity of the PVC sheet is measured a second time. The percentage obtained by dividing the second retroreflection measurement by the initial measurement is termed herein the xe2x80x9cPercent Retention of Retroreflection.xe2x80x9d
The polymeric component in the compositions and articles of the present invention comprises PVC but may also contain other polymeric resin constituents. However, the resin component preferably consists essentially of polyvinyl chloride given the superior properties thereof in sheeting applications for reflective road signs and the like where flexibility, durability and good optical properties are required. The PVC is preferably solventless PVC meaning that the PVC is capable of being processed into a film of the present invention, whether through extrusion or calendaring, without the use of a solvent. The invention however does not exclude PVC films which are cast using a solvent.
PVC used to prepare the compositions and articles of the present invention using calendaring techniques preferably has an intrinsic viscosity (xe2x80x9cI.V.xe2x80x9d) in the range of about 1.02 to about 1.14, and preferably in the range of about 1.08 to about 1.10. It should be recognized however that PVC resins having IV below about 1.02 are nevertheless within the scope of the invention. Other things being equal, the use of lower I.V. resins can detract from the retroreflectivity retention of a PVC reflective sheet. However, those skilled in the art will recognize that the lesser rigidity resulting from use of lower I.V. resin may be offset by an appropriate choice of polymeric plasticizers, fillers and other additives. On the other hand, PVC resins above about 1.14 I.V. become increasingly difficult to process and exhibit increasingly poor cold flexibility. Examples of PVC resins that are suitable for use in the present invention are the Georgia Gulf 2100, 2110 and 2095 resins.
The total amount of PVC resin in a flexible sheet according to the present invention is in the range of about 65 to about 80 weight percent, and preferably in the range of about 70 to about 75 weight percent based on the total weight of the formulation used to fabricate the sheet.
In a preferred aspect of the present invention, a PVC sheet for flexible sign applications which combines both a PVC resin having IV in the preferred range of about 1.08 to about 1.10, with use of a polymeric plasticizer mixture constituting about 30 to about 35 part per hundred parts by weight of the resin, provides an excellent balance of properties from the standpoint of film processibility, cold flexibility and retention of retroreflectivity. While the use of the polymeric plasticizer, per se, can enhance the retention of retroreflectivity in a PVC retroreflective sheet according to the invention, the additional step of using a higher IV PVC resin (preferably in the range of about 1.08 to about 1.10) results in even better retention or reflectivity. The combination of PVC with polymeric plasticizer makes possible for the first time a PVC retroreflective sheet having greatly enhanced retention of reflectivity (in comparison to sheets using monomeric plasticizer). Preferably the PVC sheets of the present invention exhibit and Average Percent Retention of Retroreflection (as determined herein) of about 60% or greater.
In order to achieve a PVC roadway sign having maximum useful life, the PVC film used to fabricate the sign can utilize both the fluorescence-stabilizing aspect of the present invention (i.e., the combination of benzophenone and HALS) and the retroreflectivity-enhancing aspect thereof (use of a polymeric plasticizer with optional higher IV resin). Nevertheless, it should be understood that each of these enhancements may be practiced independently of one another. Accordingly, in addition to reflective PVC fluorescent sheeting for road signs and other applications, the invention also encompasses fluorescent PVC articles which do not have reflective elements as well as retroreflective PVC articles which do not incorporate fluorescent dyes.
In addition to the unique combination of perylene imide dyes, plasticizers, UV absorbers and HALS discussed above, other conventional processing aids for PVC fabrication may be used such as fillers, heat stabilizers, lubricants, etc. The present invention does not require any special measures with respect to these additional conventional processing additives, which, when added in their normal amounts to a PVC resin formulation have not been found to impact the features of the present invention.
Polyvinyl chloride films according to the present invention may be made by any of the well-known techniques in the art, including calendaring a suitable PVC formulation into films having nominal thicknesses which are typical for retroreflective sheets, i.e., 7-13 mils and preferably about 8-10 mils. PVC film thickness may vary with the particular application. If the film is to be used for retroreflective applications it may be further processed in a well known manner to form a plurality of retroreflective elements on the sheet such as, for example, corner cube microprisms, four-sided prisms, Fresnel lenses, rounded lenses, and microspheres. Reference may be had to U.S. Pat. Nos. 5,156,863; 4,486,363; 4,601,861; 4,505967; 5,316,838; and 4,025,159, and WO/9920688 (published Apr. 29, 1999) all of which are incorporated by reference herein in their entirety.
The present invention contemplates fluorescent reflective sheeting for road sign applications where there is no need for additional UV-protective or UV-filtering Nevertheless, the invention does not exclude flexible road signs in which such protective layers are used to provide further enhancements in fluorescence longevity. An example of the use of such protective layers can be found in PCT application WO/9948961 (published Sep. 30, 1999) incorporated herein by reference in its entirety.
The following examples are provided to illustrate different embodiments of the invention and are not intended to limit the scope thereof as defined in the appended claims.